Small cat is an airy killer (4)
I believe the answer is:
smog
'an airy killer' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'small cat' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cat' becomes 'mog' (informal British term).
's'+'mog'='SMOG'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for smog that I've seen before include "Industrially polluted air" , "Smokey fog" , "Smoky fog" , "Pea-souper" , "Visible air pollution" .)